The present invention relates generally to semiconductor devices and, more particularly, to activation of graphene buffer layers on silicon carbide by ultra low temperature oxidation.
Graphene refers to a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a planar graphene structure does not exist in a free state, being unstable with respect to formation of curved structures such as soot, fullerenes, nanotubes or buckled two dimensional structures. Free standing graphene films have been made, but they may not have the idealized flat geometry. However, a two-dimensional graphene structure has been demonstrated on a surface of a three-dimensional structure, for example, on the surface of a silicon carbide (SiC) crystal.
Structurally, graphene has hybrid orbitals formed by sp2 hybridization. In the sp2 hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp2 orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals which exhibits a stabilization that is stronger than would be expected by the stabilization of conjugation alone, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene is not an allotrope of carbon since the thickness of graphene is one atomic carbon layer i.e., a sheet of graphene does not form a three dimensional crystal. However, multiple sheets of graphene may be stacked. A typical graphene “layer” may comprise a single sheet or multiple sheets of graphene, for example, between 1 sheet and 10 sheets.
Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions having an effective mass of zero and moving at the effective speed of light of ceff≈107 m/sec. Their relativistic quantum mechanical behavior is governed by Dirac's equation. As a consequence, graphene sheets may have a large carrier mobility of greater than 60,000 cm2/V-sec at 4K. At 300K, the carrier mobility can be about 15,000 cm2/V-sec. Also, quantum Hall effect has been observed in graphene sheets.
A perfect graphene structure consists exclusively of hexagonal cells. Any pentagonal or heptagonal cell constitutes a structural defect. It should be noted that a large number of ordered defects in the graphene structure convert a graphene layer into other carbon-based structures such as a large fullerenes and nanotubes, etc. In particular, carbon nanotubes may be considered as graphene sheets rolled up into nanometer-sized cylinders due to the presence of defects. A fullerene, also known as a “buckyball” having a pattern similar to the pattern on a soccer ball, would be formed if some hexagons are substituted with pentagons. Likewise, insertion of an isolated heptagon causes the sheet to become saddle-shaped. Controlled addition of pentagons and heptagons would allow a wide variety of shapes to be formed.
Graphene layers may be grown by solid state graphitization, i.e., by sublimating silicon atoms from a surface of a silicon carbide crystal, such as the (0001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer. Graphene layer on another material is known in the art. For example, single or several layers of graphene may be formed on a silicon carbide (SiC) substrate by sublimation decomposition of a surface layer of a silicon carbide material.
Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers.